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Re: Scalars and Matrices:

From: Francesco Potorti`
Subject: Re: Scalars and Matrices:
Date: Fri, 15 Oct 1999 13:56:35 +0200 (CEST)

(Ted Harding) <address@hidden> writes:
   > Sorry, I don't think I can sympathise with this. As a general convention
   > for associativity,
   >   A*B*C = (A*B)*C
Johan Kullstam <address@hidden> writes:
   no. associativity says, by definition, that
     (A*B)*C = A*(B*C)             (A)

What  Ted means (I  suppose) is  that, in  a programming  language, when
dealing  with non-associative  binary  operators (like  the product  for
matrices), there is a commonly used default assumption, unless otherwise
stated in the language definition.

This assumption  is that the language's  operator is "left-associative",
that is, grouping is done from left to right.

This  is quite common  and, in  my opinion,  intuitive.  I  think Octave
should not change anything in the way it deals with these ambiguities.

This is what the Octave manual says about this issue:

    When operators of equal precedence are used together, the leftmost
 operator groups first, except for the assignment and exponentiation
 operators, which group in the opposite order.  Thus, the expression `a
 - b + c' groups as `(a - b) + c', but the expression `a = b = c' groups
 as `a = (b = c)'.

Possibly, if someone  thinks so, some text could be  added to the Octave
manual which clarifies this issue  further.  If John Eaton does not feel
like  adding that text  himself, I  am sure  he will  be glad  to accept
reasonable contributions.

Francesco Potortì (researcher)         Voice:    +39-050-593 203 (op. 211)
Computer Networks Group                Fax:      +39-050-904052
CNUCE-CNR, Via Santa Maria 36          Email:    address@hidden
56126 Pisa - Italy                     Web:

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